The methylamines, mono-, di- and trimethylamine, are commodities in the chemical industry, widely used as starting materials in the preparation of other chemicals.
The methylamines can be prepared by several methods. The most common of these is the continuous catalytic reaction of methanol or dimethyl ether and ammonia, using a silica-alumina dehydration catalyst containing a low proportion, i.e., 10-15% by weight, of alumina.
While such a catalyst is generally satisfactory, it has a tendency to coke, which makes it necessary to replace the catalyst more frequently than is desirable. By "coke" is meant the phenomenon by which the surface of the catalyst becomes coated with carbon, thus blocking its pores and reducing its effectiveness.
It has now been found that this coking can be minimized if, instead of the conventional catalyst containing a low proportion of alumina, one uses a catalyst containing a high proportion, i.e., 87-99%, by weight, of alumina. Surprisingly, use of such a catalyst according to the invention not only reduces the amount of coking but also increases the rate of the methanol/dimethyl ether-ammonia reaction over the rate obtained with the conventional low alumina catalyst and reduces the amounts of amine byproducts formed, especially N-methylisobutylamine, N,N,-dimethylisobutylamine, ethylamine, propylamine and N-methylpropylamine. In addition, the catalysts of the invention have better thermal stability than the conventional low-alumina catalyst, which makes them effective for a longer period.